1. Field of the Invention
The present invention relates to a steam blower box for the application of steam on a material web, particularly paper, which travels past the steam blower box. The steam blower box includes a housing wall which faces the material web and extends transversely of the web travel direction, wherein a plurality of steam outlet openings are arranged in the housing wall.
2. Description of the Related Art
In the manufacture of paper or the further processing of paper, steam is frequently admitted to the running web. There are various reasons for this application of steam. However, the object is always to condensate the steam emitted from the steam blower box as precisely as possible on or in a certain area of the web, and to transfer heat and/or moisture to the web in this manner. Differing quantities of condensation transversely of the web make it possible to influence certain properties, such as gloss, smoothness, etc., in a targeted manner and to achieve a profiling of the web or the elimination of undesired differences of these properties across the web.
However, a maximum efficiency of the steam application is only achieved under ideal conditions, i.e., if no additional heat transfer resistance between the steam and the web is present. This ideal condition is practically never achieved. Contrary to heat exchangers or boilers, the heat transfer resistance plays no role in a running web as a result of the condensate film which is created because this condensate film is continuously moved together with the web out of the steam application area. However, the steam application space at a running web is normally not only filled with steam. Because of friction, an air film is continuously conveyed together with the web into the steam application area. An energy exchange takes place between this air and the steam. A portion of the steam condenses in the air instead of at the web and, therefore, can no longer contribute to the desired heating and moisture enrichment of the web. In addition, the air which is heated and enriched with water becomes oversaturated during cooling and precipitates water droplets. The resulting water vapors negatively affect the climate within the work area and results in the formation of droplets at machine components. Accordingly, the principal disadvantage of the presence of air is the reduced efficiency of the steam application. Since the air film screens portions of the web from direct contact with the steam, generally only a portion of the web comes into a direct heat exchange within the steam application area.
In order to overcome this disadvantage, so-called high-speed steam blower boxes have already been proposed. In these high-speed steam blower boxes, the steam jets are emitted with such a high speed that the air is displaced from the web surface at the points of impingement of the steam on the web, and a direct contact occurs between the steam and the web. For this purpose, the impinging steam jet must produce an appropriate blocking pressure depending on the web speed. At high web speeds and with acceptably large outlet openings, the necessary blocking pressure is achieved with the emitted steam only when steam quantities are applied which usually exceed the absorption capability of the web. Consequently, in the case of relatively low steam requirements, for example, when applying steam on paper webs for increasing the smoothness at calendars, it is necessary to operate with a steam excess which is much too large, in order to approach the desired condition. Also, it is frequently necessary to suction off this excess steam, wherein a high suction power is required, on the one hand, and, on the other hand, there is the danger that an unintentional suctioning off of steam portions will reduce the efficiency of the steam application.
DE 37 01 406 C2 discloses special sealing zones which have the purpose of using the direction of the steam jets for achieving a blocking effect on both sides of the steam application area. It is intentionally accepted air that is suctioned off because of the injection effect of the steam jets and the air is utilized for the sealing effect. A separation of air and steam area does not take place.
DE 44 01 220 C1 describes a steam blower box in which a sectionally different steam application can be utilized for influencing the transverse moisture profile of the material web. The sectional zone chambers used for this purpose are connected through a separate steam line and a control valve each to the steam distributor. The common steam outlet panel has several rows of blower openings. These rows of blower openings are arranged in the steam outlet panel completely inclined relative to the travel direction of the material web. The steam is directed in a direction opposite the material running direction. This steam blower box makes it possible to apply steam block by block transversely of the travel direction of the material web.